Depression is a major health problem in the United States. Twenty percent of Americans will experience an episode of depression sufficient to warrant diagnosis and treatment, and 3-5% of the population may require hospitalization. Although recent pharmacologic advances in the treatment of depression have been very successful, the use of medications is limited because not all patients respond to drugs and because drugs may induce unwanted side effects that reduce the effectiveness of the drug and increase the risk of non-compliance. The major aim of this project will be to test the hypothesis that a program of regular aerobic exercise will reduce depression in older patients with major depression. Two hundred sixteen older adults age 55-80 years of both sexes, classified as mild- moderately or severely depressed, will be randomly assigned to exercise, drug alone (nortriptyline), or drug and exercise conditions. At entry, after the completion of four months of treatment, and at six months follow- up, subjects will undergo physiological and psychological evaluations. Cardiovascular function will be assessed by symptom limited exercise testing with expired gas analyses. Depression will be assessed by standard clinical interviews and self-reports of symptoms employing well-established psychometric questionnaires. In addition, cognitive function will be assessed by clinical neuropsychological procedures including measures of attention, concentration, and memory. All subjects also will undergo a follow-up assessment six months after the completion of the treatment program. It is hypothesized that patients who are mild-moderately, but not severely depressed, who undergo aerobic exercise training will exhibit significant improvements in depression that are comparable to patients receiving drug therapy alone. In contrast, we hypothesize that severely depressed patients will not benefit from exercise alone; however, it is hypothesized that severely depressed patients who receive combined drug and exercise will have lower rates of relapse of depression at six month follow-up compared to patients who receive drug therapy alone. The data generated from the study will have important practical significance by determining the extent to which a simple behavioral intervention like exercise can improve athe psychological well-being of older depressed individuals. The data also will have important scientific significance through its potential to expand knowledge regarding the relationship of behavioral and clinical assessments of depression and the extent to which exercise training may provide benefit to older adults suffering from major depression.